alleryStart learning →Why is a first pregnancy with an Rh positive fetus rarely affected in an Rh negative mother?
AThe fetus makes its own anti-Rh antibodies
BMaternal antibodies cannot cross a placenta
CThe fetal Rh antigen appears only at term
DFetal cells rarely cross into the mother
Answer & Solution
Correct answer: D. Fetal cells rarely cross into the mother
1. The mother can only make anti-Rh antibodies after she meets the Rh antigen.
2. During a first pregnancy the Rh positive fetal cells rarely cross the placenta.
3. Without that exposure the mother is not sensitised and produces no anti-Rh antibodies.
4. Exposure typically happens instead during or immediately after the birth itself.
5. By then the first baby has been delivered, so it escapes the antibodies its own delivery provoked.
6. A second Rh positive pregnancy is the one at risk, because the antibodies now already exist and can cross into the fetal bloodstream.
7. Maternal antibodies clearly can cross the placenta, so that option contradicts the very mechanism that makes the second pregnancy dangerous.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.6 Blood Typing_
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